Each cell in the human body contains 46 different chromosomes, large units of DNA that encode instructions for that cell to grow, divide, and carry out its specialized functions. During mitosis, when a cell divides, each of these chromosomes must be accurately distributed to the two new daughter cells. If this process occurs incorrectly for even a single chromosome, the resulting daughter cells will lose or gain thousands of genes and the instructions that they contain. This type of error in chromosome segregation can result in the death of the cell and is thought to contribute to tumorigenesis. Indeed, more than 70% of tumors are observed to have abnormal numbers of chromosomes. In addition to errors that alter whole chromosome numbers, in cases where the cellular machinery makes inappropriate attachments to the chromosomes, this can result in chromosome fragmentation during cell division. These errors have been shown to cause chromosomal rearrangements, which also have the potential to result in cellular transformation and tumorigenesis. To facilitate the segregation of DNA during mitosis, chromosomes must generate physical attachments to rod-like polymers termed microtubules that provide the structure and forces to move the chromosomes. A key player in chromosome segregation is a large proteinaceous structure termed the kinetochore that forms the interface between chromosomes and microtubules. Inhibition of kinetochore activities is predicted to target cancer cells while avoiding the dose-limiting neuronal toxicity associated with microtubule-binding chemotherapeutics. Determining the molecular basis for kinetochore function is crucial to understand the defective processes that can give rise to tumor cells, and to evaluate the best targets for the diagnosis and treatment of disease. The proposed work will analyze the mechanisms by which kinetochores interact with spindle microtubule polymers in human cells. We will take parallel cellular and biochemical approaches to analyze the key proteins that bind to microtubules at kinetochores. A key focus of this work will be not only to analyze the functions and activities of the individual proteins, but also to test how the multiple different proteins that are present at kinetochores act together in a integrated manner to form robust interactions with microtubules.